1. Field of the Invention
The present invention relates to a control circuit for a cooling fan. More particularly, the present invention relates to a control circuit for a cooling fan that detects variation in temperature and controls the cooling fan's rotational velocity.
2. Description of the Related Art
Various electrical and electronic devices employ a cooling fan to cool heat generated by the devices when they operate. Such devices typically consist of information processors such as a computer, office automation (OA) appliances, home electronic appliances, electric power equipment, and so on. Typically, the motor shaft rotation rate and the cooling fan airflow rate are determined in consideration of the expected temperature of the device. Thus, the cooling fan is designed to operate at a predetermined velocity. Such a static velocity profile, however, results in lower cooling efficiency and an increase in noise in general.
To improve the problems with efficiency and noise, conventional cooling fan control circuits are designed to change the cooling fan driving velocity according to variations in temperature, as shown in FIG. 4.
The conventional cooling fan control circuit shown in FIG. 4 consists of a temperature detector made of a thermistor TH for detecting temperature and temperature compensation resistors R1 and R2; comparators COM 1 and COM 2 having as input the detecting voltage which changes according to variation in temperature and connected to the noninverting terminal and a predetermined standard voltage connected to the inverting terminal; and a switch TR for applying the input voltage to the cooling fan as a function of comparator output.
In the conventional control circuit of FIG. 4, the cooling fan is driven at a low velocity—predetermined rotation number (a)—to reach temperature t1 (FIG. 5) even though the resistance value of thermistor TH changes in response to the temperature variation. Each of the comparators COM1 and COM2 is prevented from operating and the switch TR maintains a disabled state until the temperature detected by the temperature detector reaches the predetermined level t1 (FIG. 5). The input electric power +Vcc causes current to flow through output resistor R5 and maintains the predetermined rotation number (a) by supplying a predetermined voltage to the cooling fan.
When the temperature reaches a first level t1, the resistance value of the thermistor TH is lowered and the voltage at the input of the noninverting terminal of comparator COM 1 increases. Similarly, the voltage is divided by the temperature compensation resistors R1 and R2 so that the voltage at the noninverting terminal of comparator COM2 increases as well. Voltages at the noninverting terminals of COM1 and COM2 vary relative to the standard voltage divided across resistor R4 and provided as input to the inverting terminals of COM 1 and COM2. Accordingly, as comparator COM 1 operates, base electric current of the switch TR increases, and a first output voltage provided by the collector terminal supplies electric power to the cooling fan. As a result of the increase in temperature, and corresponding increase in voltage to the cooling fan, the first output voltage maintains a predetermined rotation number (b), which is an increase from the previous rotation number due to the higher voltage, relative to the distribution voltage supplied by output resistor R5.
If the temperature increases from the first level t1 to a second level t2, the voltage input to comparator COM 2 rises relative to the voltage distributed to the temperature compensation resistor R2 at the first temperature level t1. As comparator COM2 operates, the base current of switch TR increases. The output from the collector is higher for the second temperature level t2 than for the first temperature level t1 and is supplied to the cooling fan. Accordingly, as the comparator COM 2 operates, the rotation number of the cooling fan maintains a predetermined rotation number (c), which is an increase from the previous rotation number (b) caused by the second output voltage due to operation of comparator COM 1.
Thus, the control circuit of the cooling fan in FIG. 4 variably controls the rotation of the cooling fan according to variation in temperature. However, as shown on the graph in FIG. 5, the rotation of the fan drastically changes from (a) to (b) to (c) in response to temperature increase. This in turn requires an increase in the output voltage supplied to the cooling fan, which increases the amount of noise generated by the cooling fan.